PC-Monitoring

Monitoring

       Goals and Objectives
       Questions and Hypotheses
       Methods

Over the course of a 5-7 year period, much of the interannual variability within the CCS may be related to the ENSO cycle. Examining the biological response to ENSO-related physical variability provides an approach for identifying and quantifying biophysical interactions within the CCS. The intensive field studies will provide detailed "snapshots" of the biophysical interactions, but unless they are extremely fortuitously timed, the field studies will not resolve the full evolution of biological responses to an ENSO cycle. While some features of the biological responses and physical causes are relatively well known, the study of the biophysical linkages has been piecemeal in the past, relying on observation systems existing for other purposes. Present monitoring of the conditions of the CCS consist mostly of surface or coastal measurements and are primarily of physical parameters only. Existing monitoring may be sufficient to provide the rough physical context of the ENSO cycle, but it is clearly not adequate for monitoring the biophysical interactions that occur. The CalCOFI surveys in Region III off Southern California provide an important exception. To place the process studies within the context of the interannually varying biophysical interactions, occuring over the 5-7 year period, it will be necessary to augment monitoring of both biological and physical measurements, especially in regions other than the Southern California Bight.

As in most coastal ocean locations, a number of physical parameters are measured on a regular basis along the west coast of the U.S., providing a good basis for designing an augmented monitoring system. These include sea level at various tide gauge stations, surface meteorology (met) over coastal land stations and at buoys 10-20 km offshore (over the shelf), SST and wave statistics at other nearshore buoys, coastal SST and salinity at selected locations, and surface met and SST from merchant vessels. Subsurface temperature has been measured from a small number of merchant vessels along a few ship tracks in the past, but this activity has decreased in the last decade. Under cloud-free conditions, NOAA satellites measure SST four times each day and the SeaWiFS color sensor will provide an estimate of surface pigment concentrations every two days. At present sea surface height is measured by altimeters along tracks separated by 100-300 km, which can be used to estimate the large-scale surface circulation patterns starting 50 km offshore, with one realization every 10-35 days. Satellite scatterometers provide estimates of wind stress fields starting 50 km offshore, with resolution of 50-100 km every 1-4 weeks. Existing weather forecast models provide estimates of surface wind stress and heat fluxes over the ocean every 6-12 hours, with horizontal grid spacings of 100-500 km. The value of these measurements to U.S. GLOBEC EBC is limited, because they are: (1) almost entirely physical; (2) made predominantly at the surface; and (3) either shore-based or provide only very coarse resolution off-shore.

The suggested biological-physical interaction monitoring should augment existing monitoring programs. Most of the augmentation will deal with specific biological responses to physical variability at daily to interannual time scales and mesoscale phenomena at the coast and at the core of the California Current.

Goals and Objectives

The overall goal is to build a new augmented set of measurements in support of the U.S. GLOBEC EBC program at the mesoscale, seasonal and interannual time scales through regular monitoring. To interpret the latitudinal gradients and interannual scales, measurements in each of the four regions should begin as soon as possible. Standardized methods should be used to permit comparison now among regions and through time. Several approaches to implement better monitoring of the CCS are being considered:

Increase the number of parameters measured from meteorological buoys over the shelf to include subsurface measurements (e.g., temperature, conductivity, ADCP water velocity, acoustically derived plankton patchiness and biomass, chlorophyll, nutrients).
Establish a network of nearshore biological sampling with uniform methods (such as settlement plates) to estimate rates of retention, advection and recruitment of benthic invertebrates.
Establish several buoys similar to those over the shelf but located offshore of a few selected shelf buoys to independently describe shelf and deep water forcing and dynamics and to estimate cross-shore gradients of wind stress (the most important component of the curl during spring and summer) and zooplankton biomass and patchiness (using acoustics).
The CalCOFI program and the Canadian La Perouse Bank Study provide seasonal subsurface biophysical data in Region III and at the northern end of Region I, respectively. Augment this monitoring with a small number of offshore transects in Regions I and II, while encouraging a similar effort off Baja California, Mexico (Region IV). Measure key physical (e.g., T, S, water velocity) and biological (e.g., size-specific phytoplankton production, chlorophyll, zooplankton species abundance and production, and fish egg production) parameters over the upper several hundred meters, extending several hundred kilometers offshore, with seasonal coverage. Use chartered vessels or research vessels-of-opportunity for more frequent biological sampling on the inner portion of the transects.
Establish a program of seasonally-released drifters to build up Lagrangian statistics on the circulation of the CCS within which annual migrations and recruitment of marine populations take place.
Establish a network of moored sediment traps to maintain a long time series of selected variables of the biological response to climate change, and to calibrate the long-term sediment record in time and space.
Intensify the merchant ship XBT program along a few representative lines.

Several planned activities within NOAA may help accomplish these objectives. The addition of downward looking ADCP and/or subsurface temperature sensors to existing National Data Buoy Center (NDBC) buoys has been discussed at NDBC. Prototypes of these buoys should be deployed in the regions chosen for long-term monitoring and intensive process studies (which should coincide) in the U.S. GLOBEC CCS study. The connection between the California Current System and the tropical Pacific should be explored using measurements collected by the Tropical Ocean Global Atmosphere (TOGA) program and its successor, Global Ocean Atmosphere Land System (GOALS) program. It would be ideal if more observational elements (tide gauges, ATLAS moorings, etc.) could be located along the eastern margin of the Pacific to explore the oceanic connection between the tropics and the Pacific EBCs, especially the CCS. This would provide a data set with which to validate the interplay between the nested regional models and the basin-scale GCM.

Questions and Hypotheses

The monitoring component addresses the questions of local and distant forcing directly, in addition to these general questions:

What are the natural modes of biological and physical variability at interannual and longer time scales?
Where do the spatial surveys and other process study measurements collected as part of the mesoscale and large-scale studies during the U.S. GLOBEC 5-7 year EBC project fit into the basin-scale ENSO cycle and longer period modes of variability in the CCS?
Can we distinguish anthropogenic impacts from natural variability at some point in the data record?

These questions assume, and we recommend, that the monitoring continue beyond the end of the 5-7 year U.S. GLOBEC study, in order to define the longer term variability, to provide the data needed to drive and validate the ecosystem models, and to provide the system for detecting climatic changes.

Methods

Monitoring the biological response to climate change. Regular biological and hydrographic surveys in Regions I (La Perouse Bank) and III (CalCOFI) must be augmented in Regions I, II and possibly IV. Pelagic and demersal fish and target invertebrates should be surveyed seasonally to accompany the offshore drifter and buoy data. Biological data should be collected on along- and cross-shelf transects. Standard concurrent CTD sampling includes bottle sampling at a number of depths. Towed, undulating vehicle surveys may only include chlorophyll florescence with depth, but surface "flow-through" sampling may be possible that will still contribute biological information. Eventually, acoustic and optical instruments, and chemical sensors will be available to measure biological variables in an automated fashion. Small NOAA or chartered vessels should be used to provide more frequent biological sampling over the shelf. An inexpensive network of shore monitoring stations associated with government or academic institutions should be established and funded long-term to collect both physical and biological data. A shore station with meteorological, tide gauge, SST, salinity and biological measurements should be maintained at the inshore end of each transect and within any region routinely surveyed for abundance of larvae and adults of key species. Methods at all stations should be standardized to assure intercomparability. Because of the dynamic nature of Region II, there should be several new monitoring sections established to complement the seasonal surveys now done in Regions I, III and northern part of IV. Ideally such sites would be located where historical data, especially subsurface data, are available. To the extent possible, the different types of measurements should be co-located. For instance, the shelf and offshore meteorological buoys with subsurface measurements should lie along one of the offshore transects and the offshore buoy should lie under an altimeter track or cross-over point. If this could coincide with a merchant ship XBT line, the benefit would be increased. Efforts should be made to encourage similar and coordinated Canadian and Mexican monitoring programs.

Sediment trap data. To fully exploit the potential of the high-resolution sediment records to study climate change and its impact on marine biota, it is important to monitor the processes that create the sediment record. This is done using sediment trap moorings near the sites of deposition. Sediment trap studies provide information on how the seasonally varying input of biogenic and terrigenous material relates to changing environmental conditions, and what proxies of biological and physical processes are preserved in the sediments. Time series sediment trapping, combined with hydrographic measurements and remotely sensed observations of surface ocean conditions provide an ideal means to investigate these questions and should be an integral part of the monitoring efforts of the U.S. GLOBEC EBC program so that we may link the mesoscale and regional process studies to the seasonal, interannual, and interdecadal changes preserved in the sediment records. A spatial array of traps moored near and distant from sediment records could give an indication of the spatial variability of sedimentation, thus the degree to which the sediment time series represents historical variability on larger space scales.

Physical forcing and processes of importance to marine populations. Inflow/Outflow at the boundaries-Altimeter alongtrack heights allow the calculation of cross-track geostrophic surface velocity along tracks that are more than 20-50 km from the coast. By defining a volume inshore of a system of tracks, one can calculate geostrophic inflow/outflow at the surface. An altimeter does not sense wind-driven Ekman drift, which may be calculated from wind stress-most likely from model winds, since the scatterometer may alias storms. Thus, with data already available from satellites and/or surface buoys, surface currents can be calculated (except within 20-50 km of the coast). To examine currents over the shelf (nearshore) and vertical current shear, either CTD data or ADCP current data (at buoys) are needed. Augmenting the offshore meteorological buoys with downward looking ADCPs would be extremely valuable, especially if located under altimeter tracks or crossover points, for verifying the winds and the baroclinic shear in the upper ocean that the altimeter is missing.

Wind stress, surface mixing, wind stress curl, timing of seasonal events-The present system of meteorological buoys located 10-20 km from the coast, along with model winds, probably provide everything needed except the wind stress curl. The model winds underestimate the curl and smooth it over large distances due to their coarse spatial resolution. The cross-shelf component of the curl in spring and summer could probably be measured by placing a met buoy farther offshore (approximately 100 km) and another located only 1 km from the shore, but a denser (in the along-shore direction) array of buoys would be needed to capture the curl in winter.

Water temperature and stratification-AVHRR images, met buoys and merchant vessels all provide SST. To document subsurface temperature changes, including stratification, we need to add subsurface temperature measurements at met buoys, strengthen the XBT program and make periodic vertical slices (transects), either cross-shelf at a few locations and alongshelf at physical boundaries.

Transport-The altimeter does not sample well within 50 km of the coast and is presently unusable within 100-200 km of the coast off Oregon and Washington because of incorrect tidal corrections. Offshore CTD transects will measure the mean geostrophic transport relative to some depth and ADCP measurements will improve the calculations. Cross-shelf transects will reveal jets or eddies crossed along the way, if sampling is sufficient. Towed, undulating vehicles with CTD sensors provide the best coverage. Complete small-scale 3-D surveys are needed to define the eddy field (scales of 10-200 km), which are prohibitive for a monitoring component. Drifters provide information on the eddy field statistics and typical velocities. Presently available winds are probably adequate to describe the surface Ekman transport. Rough estimates of upwelling and vertical transport made from geostrophic wind fields are probably adequate.

Movement of regional physical boundaries-When clouds are absent, the surface signature of these boundaries sometimes may be monitored from satellite SST and color images. However, some of the regional boundaries do not have surface expressions. To describe these boundaries, an alongshelf transect (CTD, ADCP) will be most effective.

Vertical structure-Buoys may provide continuous profiles of temperature structure and velocity at a point, providing information on the forcing of internal mixing. Merchant vessel XBTs provide stratification periodically along regular routes. Regular transects can provide information about the vertical structure of chlorophyll and nutrients. The depths of the thermocline and nutricline are of particular interest, since they may change in some climate change scenarios.